Microbial degradation of dissolved proteins in seawater1

Abstract

An experimental protocol using radiolabeled proteins was developed to investigate the rates and mechanisms whereby dissolved proteins are degraded in natural marine plankton communities. The results of field observations and laboratory experiments indicate that proteins are degraded by a particle‐bound, thermolabile system, presumably bacteria‐associated enzymes, with an apparent half‐saturation constant of ca. 25 µg bovine serum albumin (BSA) per liter (ca. 1.7 × 10‒10 M). Gel permeation chromatography indicates that peptides of chain length intermediate between BSA and the final products of degradation (MW < 700) do not accumulate in the medium. Competition experiments indicate that the system is relatively nonspecific (affinity: proteins > large peptides ≫ small peptides). Turnover rates for the protein pool in samples collected in the Southern California Bight were of the same order of magnitude as the turnover rate of the l‐leucine pool and were correlated with primary productivity, chlorophyll a concentrations, bacterial abundance and biomass, and l‐leucine turnover rate. These data suggest that amino acids derived from proteins are utilized preferentially and do not completely mix with the amino acids in the bulk phase.